The present invention relates to improvement in a gas sensor having a coiled heater electrode.
A gas sensor is known wherein the surface of a metal oxide semiconductor having a coiled heater electrode is covered by a filter (for example, Japanese Patent Opening Hei 11-142356). Such a gas sensor is used, for example, by subjecting the gas sensor to periodical temperature changes, to detect CO at low temperature and a combustible gas, such as methane, at high temperature. As interfering gases, such as alcohol, can be easily removed with activated carbon filter or the like, the main interfering gas is hydrogen when CO is to be detected and when a combustible gas is to be detected. The resistance of the sensor in CO is generally higher than that of the sensor in a combustible gas, and this poses a problem in designing a detection circuit.
The objects of the present invention are
1) to bring the sensor resistance in CO and that in a combustible gas close to each other,
2) to improve the selectivity from hydrogen towards CO and combustible gases,
3) to increase the concentration dependency in the combustible gases, and
4) to decrease the concentration dependency in CO of low concentrations.
The present invention is a gas sensor wherein an inner area of SnO2 is provided to cover a coiled heater electrode and the inner area is covered by a filter, said gas sensor being characterized in
that the volume of the inner area is from 1xc3x9710xe2x88x923 mm3 to 16xc3x9710xe2x88x923 mm3 
and that the ratio of the total volume of the inner area and the filter to the volume of the inner area is from four to twenty.
Preferably, the volume of the inner area is from 3xc3x9710xe2x88x923 mm3 to 10xc3x9710xe2x88x923 mm3, the ratio of the total volume of the inner area and the filter to the volume of the inner area is from four to fifteen, and the total volume of the inner area and the filter is from 15xc3x9710xe2x88x923 mm3 to 70xc3x9710xe2x88x923 mm3.
More preferably, the volume of the inner area is from 4xc3x9710xe2x88x923 mm3 to 10xc3x9710xe2x88x923 mm3, the ration of the total volume of the inner area and the filter to the volume of the inner area is from five to fifteen, and the total volume of the inner area and the filter is from 30xc3x9710xe2x88x923 mm3 to 60xc3x9710xe2x88x923 mm3.
Preferably, a central electrode is provided at the center of said heater electrode.
Preferably, both the filter and the inner area contain SnO2, the filter contains an aggregate, such as alumina, silica and zeolite, and the content of the aggregate in the filter is equal to or higher than that in the inner area. The content of the aggregate in the inner area may be zero.
Preferably, the concentration of a precious metal catalyst such as Pd and Pt, in the filter is lower than that in the inner area, and in the extreme case the filter may not contain any precious metal catalyst.
The present inventor examined how three factors, namely, the volume of the inner area, the total volume of the inner area and the filter, and the ratio of the total volume to the volume of the inner area, influence the characteristics of the gas sensor. The objectives were to bring the sensor resistance in CO and that in a combustible gas close to each other, to increase the selectivity from hydrogen towards CO and combustible gases, to increase the concentration dependency in combustible gases, and to decrease the concentration dependency in CO of low concentration (for example, from 100 ppm to 300 ppm). The concentration dependency in CO of low concentration is generally too high. For example, the resistance in CO of 300 ppm is 1/20 or lower of the resistance in CO of 100 ppm, and as the concentration dependency is too high, it is hard to design a driving and detecting circuit.
We have found that the volume of the inner area and the ratio of the total volume to the inner area influence the sensor characteristics at low temperature (characteristics for CO detection), and that the total volume influences the sensor characteristics for combustible gas detection. As for the inner volume and the volume ratio, when the volume of the inner area is small and the volume ratio is high, the resistance in CO is low and is closer to the resistance in the combustible gases, and the selectivity from hydrogen increases and the concentration dependency on CO at low concentrations decreases. On the basis of the values of these factors of the gas sensors produced on a trial basis, the volume of the inner area was set to be from 1xc3x9710xe2x88x923 mm3 to 16xc3x9710xe2x88x923 mm3, preferably from 3xc3x9710xe2x88x923 mm3 to 10xc3x9710xe2x88x923 mm3, and more preferably from 4xc3x9710xe2x88x923 mm3 to 10xc3x9710xe2x88x923 mm3. Similarly, the ratio of the total volume to the volume of the inner area was set to be from four to twenty, preferably from four to fifteen, and more preferably from five to fifteen.
The sensor characteristics in the combustible gases depend, as describe above, on the total volume. It was found that when the total volume is reduced, the sensor resistance in the combustible gas will decrease, but the selectivity from hydrogen will increase, the concentration dependency will increase, and at the same time the sensitivity to the combustible gases will increase. On the basis of these findings, the total volume was set to be from 15xc3x9710xe2x88x923 mm3 to 70xc3x9710xe2x88x923 mm3, and preferably from 30xc3x9710xe2x88x923 mm3 to 60xc3x9710xe2x88x923 mmxe2x88x923.